How Overhead Line is Drawn? | Electricity | Electrical Engineering

The following article will guide you about how overhead line is drawn, explained with the help of suitable diagrams.

The route along which the overhead line is to be drawn should at first be surveyed and one peg is to be driven into the ground wherever a pole is to be erected. It is necessary to watch that the poles are erected at almost equal intervals and that there is no unnecessary bend in the line. Besides, it should also be borne in mind that according to Indian Electricity Rule No. 85, the length of span between two consecutive poles of a low or medium voltage line shall not exceed 67.056 metres (220 ft).

After survey an earth pit is made at each point indicated by a peg and later a pole is erected there. The depth of the pit depends on the length of the pole, the nature of soil and the position of the pole. In case of marshy land or sandy soil or soft soil, earth pit should be deeper. But in case of rocky soil the pole remains erect and stable in a pit having comparatively less depth.

Again, a terminal pole or a tangent pole is subjected to more tension than other poles. Hence, for these poles also earth pits should be deeper. Normally, however, it is considered sufficient if one-sixth of the total length of a pole in a low or medium voltage line remains within earth pit.

Diameter of the pit depends on diameter of the pole. Around a pole it is the general practice to keep a clearance of 15 cm to 23 cm between surface of the pole and the pit wall. A pit can be made in soft soil by means of spade, crowbar or anger. Sometimes, however, it may be necessary to use drilling rig for rocky soil.

If the soil is soft, the pole may set itself too for inside the pit due to pressure from above as well as due to its own weight. To remove this trouble often a cast iron base plate is attached to the bottom of the pole. In some places a concrete slab of thickness 7.5 cm (3″) is cast inside and at the bottom of the pit.

The usual method of erecting a pole within an earth pit involves physical labour and skill. In the process are required four pieces of substantial ropes each 15 metres (50 ft) long, a piece of thick wooden plank and two pieces of bamboo tied cross-wise to each other in the form of a pair of scissors.

At first four pieces of rope are very tightly bound together at the top of the pole. The pole is then brought beside the hole. At this stage the bottom of the pole is made to rest on the piece of plank so that it does not slip at the time of raising it for erection.

A few people together should then raise the top of the pole about 1.5 to 1.8 metres (5 to 6 ft.) high from ground level and make it rest on the pair of bamboos joined together in the shape of pair of scissors. Now, three pieces of rope are kept in tension in three different directions and the fourth one is pulled slowly by a few people together so as to place the pole in erect position within the pit with the help of plank.

After making the pole erect it is necessary to check whether its position is in line with other poles. If other poles are not yet erected, man with a long staff in hand should be placed by the side of each of three or four consecutive pegs locating the other poles. The position of the pole under erection is correct if it is in line with these staffs.

While location of a terminal pole is checked with others on only one side of the pole, that of an intermediate pole should be checked with others on both sides of its location. Otherwise the probability of the line becoming curved is not ruled out. Whether the pole is properly erect can be checked by means of a plumb or spirit level.

To keep a pole properly erect for long, its foundation, of necessity, must be very strong and durable. In some places the space around the pole inside the pit is stuffed with soil and pieces of pumice-stone. In such cases the pit is stuffed with these materials in parts, and every time these are duly rammed. But wherever the soil is tender, the best arrangement is to fill up the pit with cement concrete mixture.

For preparing concrete mixture cement, sand and stone chips are to be taken in the ratio 1:2:4 respectively and then mixed together with the help of suitable quantity of water. After the pit is filled up, the foundation of concrete should project upward at least 30 cm (1 ft.) above the ground level. This upper portion should be so slanting as not to allow rain water to accumulate on it.

After erection stay wires are to be fixed up with terminal poles and tangent poles (i.e. a pole erected at a point of bending of the line). In case line conductors are drawn while these poles are not yet fixed with stay, the tension of the conductors may deflect the poles. The stay wires, however, should not be kept taut until the line conductors are also drawn; otherwise tension in the stay wires may deflect such poles.

After the line conductors are drawn and simultaneously with the adjustment of sag and tension of the lines, the tension in the stay wire is also adjusted either by turning the bow over the stay rod or with the help of a straining screw where such screw has been used with the stay-set.

Immediately after fixing the stay, according to configuration of the line, cross-arms or D-iron clamps are to be fixed to each pole. Next, the line conductors are drawn. At first the drum on which the wire is coiled is to be so mounted on a shaft which is raised off the ground by means of two jacks on both sides that the drum can turn freely.

At the time of uncoiling the wire, it should be carefully observed that the uncoiling takes place from the upper part of the drum. This reduces the rubbing on the wire. Later on a grooved pulley is fitted to every cross-arm or D-iron clamp and the wire is drawn through the groove of the pulley. Copper wire may be drawn through an iron pulley, but for aluminium wire pulley must be made of aluminium.

Often the pulley-block is not handy while drawing low or medium voltage line. In that case copper wire may be drawn directly over the cross-arm or through the D-iron clamp. But aluminium is rather soft metal which on rubbing with iron bracket or clamp may either snap or wear out too much.

For this reason the exact spot on the cross-arm or of the clamp through which the aluminium wire is to be drawn is wrapped up with jute cloth. This prevents any appreciable loss to the wire in-spite of rubbing when the line is drawn. While drawing a wire, special attention is to be given to watch that no joint in the. wire rests over a cross-arm or a clamp.

Next, the tension of the line and sag of the wires are to be adjusted. The special type of clamp which is used with a terminal pole for this job is known as come-along clamp. The topmost live conductor of the line is bound to the clamp head. The screw is then turned with the result that the hanging wire rises slowly and at the same time tension of the wire keeps increasing.

On reaching the height corresponding to the specified sag, the end of the wire is bound up with the shackle insulator. The come-along clamp thus freed is taken out and bound with the next line conductor. However, once the sag of the first conductor is adjusted, such adjustment is no more necessary with subsequent conductors drawn.

In such cases same distance is maintained from the first wire throughout the length of each span, and finally the wires are bound up with respective insulators. In low and medium voltage lines the distance between two conductors drawn side-by-side is about 30 cm (1 ft.).

After binding the Wires with the insulators at the terminal pole, they are set in the grooves of insulators fixed to intermediate poles and bound up properly. Finally, to complete the line, necessary safety device, cradle guard etc. are fixed in position.

At the time of drawing an overhead line certain rules and regulations laid down in Indian Electricity Rules, 1956 are to be observed.

These rules are stated in brief below:

(i) I.E. Rule No. 74(1) states that all conductors of overhead lines shall have a breaking strength of not less than 317.51 Kg (700 lb).

(ii) Rule No. 77(1) states that for low and medium voltage line, no conductor of an overhead line, including service lines, erected across a street shall at any part thereof be at a height less than 5.791 metres (19 ft.).

(iii) Rule No. 77(2) states that for low and medium voltage line, no conductor of an overhead line, including service lines, erected along any street shall at any part thereof be at a height less than 5.486 metres (18 ft.).

(iv) Rule No. 77(3) states that for low, medium and high voltage lines up to and including 11,000 volts, no conductor of an overhead line, including service lines, erected elsewhere than along or across any street: 

(a) Shall be at a height less than 4.572 metres (15 ft.) if the conductors are bare,

(b) Shall be at a height less than 3.963 metres (13 ft.) if the conductors are insulated.

(v) According to I.E. Rule No. 79(1) where a low or medium voltage overhead line passes above or adjacent to or terminates on any building, the following minimum clearances from any accessible point on the basis of maximum sag, shall be observed-

(a) For any flat roof, open balcony, verandah roof and lean- to roof:

(i) When the line passes above the building a vertical clearance of 2.439 metres (8 ft.) from the highest point, and

(ii) When the line passes adjacent to the building a horizontal clearance of 1.219 metres (4 ft.) from the nearest point, and

(b) For pitched roof:

(i) When the line passes above the building a vertical clearance of 2.439 metres (8 ft.) immediately under the lines, and

(ii) When the line passes adjacent to the building a horizontal clearance of 1.219 metres (4 ft.).

Sub-rule No. 79(2) states that any conductor so situated as to have a clearance less than that specified in Sub-rule No. 79(1) shall be adequately insulated and shall be attached by means of metal clips at suitable intervals to bare earthed bearer wire having a breaking strength of not less than 317.51 kg. (700 lb).

Sub-rule No. 79(3) states that the horizontal clearance shall be measured when the line is at a maximum deflection from the vertical due to wind pressure.

(vi) I.E. Rule No. 78 specifies the clearance between overhead line conductors and trolley-wires.

Rule No. 78(a) states that in case of low and medium voltage lines no conductor of an overhead line crossing a tramway or trolley bus route using trolley wires shall have less than 1.219 metres (4 ft.) clearance above any trolley wire.

Provided that where an insulated conductor suspended from a bearer wire crosses over a trolley wire, the minimum clearance for such insulated conductor shall be 0.610 metre (2 ft.).

(vii) I.E. Rule No. 85 specifies the maximum intervals between supports of an overhead line.

According to this rule in case of overhead lines carrying low or medium voltage conductors, when erected in, over, along or across any street, the interval shall not, without the consent in writing of the Inspector, exceed 67.056 metres (220 ft.)

(viii) I.E. Rule No. 88 is related to guard-wire.

Rule-No. 88(2) states that every guard-wire shall be connected with earth at each point at which its electrical continuity is broken.

Rule No. 88(3) states that every guard-wire shall have an actual breaking strength of not less than 635.02 kg. (1400 lb.) and, if made of iron or steel, shall be galvanised.

According to Rule No. 88(4) every guard-wire or cross-connected system of guard-wires shall have sufficient current carrying capacity to ensure the rendering dead, without risk of fusing of the guard-wire or wires till the contact of any live wire has been removed.

(ix) I.E. Rule No. 91 specifies safety devices.

According to Rule No. 91(1) every overhead line (not being suspended from a dead bearer wire and not being covered with insulating material and not being a trolley-wire erected over any part of street or other public place or in any factory or mine or on any consumer’s premises) shall be projected with a device approved by the Inspector for rendering the line electrically harmless in case it breaks.

(x) I.E. Rule No. 92 relates the protection of an overhead line against lightning.

Rule No. 92(1) states that the owner of every overhead line which is so exposed as to be liable to injury from lightning shall adopt efficient means for diverting to earth any electrical surges due to lightning.

According to Rule No. 92(2) the earthing lead for any lightning arrestor shall not pass through any iron or steel pipe, but shall be taken as directly as possible from the lightning arrestor to a separate earth electrode subject to the avoidance of bends wherever practicable.

(xi) I.E. Rule No. 93 is related to unused overhead lines.

Rule No. 93(1) states that where an overhead line ceases to be used as an electric supply line, the owner shall maintain it in a safe mechanical condition or shall remove it.

According to Rule No. 93(2) where any overhead line ceases to be used as an electric supply line, an Inspector may, by a notice in writing served on the owner, require him to maintain it in a safe mechanical condition or to remove it within fifteen days of the receipt of the notice.

Jointing of Overhead Line Conductors:

At the time of jointing overhead line conductors, it should be kept in mind that the joint retains all the specifications and qualities of the conductor itself. I.E. Rule No. 75 states that joints between conductors of overhead lines shall be mechanically and electrically secure under the condition of operation. The ultimate strength of the joint shall not be less than 95 per cent of that of the conductor, and the electrical conductivity not less than that of the conductor.

In practice there are various methods of jointing overhead line conductors.

Of these, the more common methods adopted for low and medium voltage lines are briefly discussed below:

(1) Britannia Joint:

For jointing hard-drawn copper wires or stranded aluminium wires, Britannia joint may be used. Its use is, however, more common in jointing solid conductors rather than stranded ones.

At first the two ends of the two conductors to be joined together are properly cleaned and one is placed on the other. The length of overlap between each other depends on the size of the conductor. Usually this length varies from 15 cm up to 35 cm. The tips of the conductors at both ends of the joint are slightly bent. This prevents easy loosening and disjointing.

The two ends of conductors are then properly bound up by means of soft binding wires. Copper binding wires should be used for line conductors made of copper, while for aluminium line conductors aluminium binding wires are to be used. Copper binding wires are made of tinned copper and its size is usually 14 S.W.G.

Placing the binding wire just at the middle of the joint, it should be bound very tightly and in close compactness. A sketch of the Britannia joint is shown in fig. 286. To prevent loosening, binding wire is wound over the conductor on both sides beyond the overlap up to a length of about 4 cm. In case of copper binding wires this part is often soldered on both sides.

(2) Married Joint:

This type of joint is adopted only for stranded conductors.

At first the strands at one end of each of the two conductors are loosened by turning the strands in a direction opposite to that in which these have been wound. Next, every strand is straightened and properly cleaned. Un-cleaned strands would increase the resistance and hence decrease the conductivity of wire at the joints. For this reason special attention is necessary in regard to this matter.

In the next stage strands of one conductor are inserted into those of the other conductor and these are wound one by one manually. The appearance of the joint after winding all the strands in this manner is shown in fig. 287(b). The use of all aluminium conductors for low and medium voltage lines is more in vogue in our country. These are all stranded conductors. Hence, for jointing two conductors of an overhead line, in most cases married joint is found to be adopted.

(3) Splice Joint:

If the wires drawn in an overhead line are solid conductors, splice joint may be adopted for jointing two such wires. The two ends of two conductors are properly cleaned up to a length of 30 to 35 cm from the tip and the end of one conductor is placed on that of the other conductor. Next, one end is wound about 3 to 4 turns around the other end and then the second end is also wound 3 to 4 turns around the first end. Thus the jointing is complete.

The different stages of development of a splice joint are shown in fig. 288. The open tips of the two conductors are wound in close compactness and finally soldered.

(4) Sleeve Joint:

If the wires of an overhead lines are aluminium conductors, sleeve joint may be adopted for joining two such wires. This type of joint is possible for all kinds of wires, but it is found to be more commonly used in high voltage line rather than in low and medium voltage lines.

There is an aluminium sleeve or covering into which the ends of two conductors are inserted from opposite directions. At first the inner surface of the sleeve and the ends of the two wires are properly cleaned. The end of the each wire will enter into the sleeve through one end and will come out at the other end of the sleeve so that a little part of the tip is projected outside. After jointing this tip is to be bend.

Two persons are required for the process of jointing. After inserting the two ends of two conductors into the sleeve from opposite directions, two persons will tightly hold the two ends of the sleeve with the help of two dies or two wrenches and stand face to face with each other.

Next, one person will only hold one end tightly and the other person will twist the sleeve up to 2 ½ to 3 turns in any direction. After this second person will just hold one end and the first person will twist the sleeve up to 2 ½ to 3 turns in a direction opposite to that of the previous twist. The turns on the sleeve must, however, be continuous. Now, to complete the joint, the tips of two conductors are somewhat bent.

Sag of the Wires:

When wires of an overhead line are drawn, each wire somewhat drops from horizontal position in between two consecutive poles. The maximum droop from the horizontal line joining positions of wire at the two poles is called the sag. This happens at the mid-span if the poles are on horizontal ground. If, however, one pole is on a lower ground than the other which is on relatively high ground, the sag is shifted towards the pole on the lower ground. Fig. 290 shows the sag of an overhead conductor.

The sag of an overhead line conductor depends on span of the line (i.e. the distance between two consecutive poles), the size of the conductor and the temperature of the surrounding air. For high voltage line length of sag is at first computed according to different formulae. Afterwards when the line conductor is drawn, temperature of the surrounding air is observed and the sag is maintained according to that temperature. Otherwise there is possibility that the wire may snap later.

But so much computation is not necessary for a low or a medium voltage line. In these lines the span is very short and the conductors have comparatively smaller cross-section. Hence, wires may be drawn maintaining-some amount of sag according to a rough estimate. Besides, standard tables are available which show-different amount of sag corresponding to different span length. If necessary sag may be determined from these tables also. On the whole there is no risk in low and medium voltage lines if the actual sag is somewhat more or less than the precise value.

Earthing of Overhead Lines:

I.E. Rule No. 90 is related to earthing of an overhead line. According to Rule No. 90(1) all metal supports (i.e. supporting poles) of overhead line’s and metallic fitting attached thereto, shall be permanently and efficiently earthed. For this purpose a continuous earth wire shall be provided and securely fastened to each pole and connected with earth ordinary at four points in every 1.609 Km. (one mile), the spacing between the points being as nearly equidistant as possible. Alternatively, each support and the metallic fitting attached thereto shall be efficiently earthed.

Rule No. 90(2) states that each stay wire shall be similarly earthed unless an insulator has been placed in it at a height, not less than 3.048_t.metres (10 ft.) from the ground.

Broadly speaking there are two different methods for earthing overhead line supports i.e. poles and ancillaries: 

(i) To draw a continuous earth wire on the top of the poles throughout the length of the line,

(ii) To connect each pole and its ancillaries to a separate earth connection.

Of these two arrangements, earthing of an overhead line by drawing a continuous earth wire along the line is comparatively good arrangement; because this reduces the earth resistance to a low value and protects the live lines and the neutral from being damaged due to lightning, as the lightning strikes the topmost earth wire first and is immediately discharged to earth.

The main purpose of earthing is that, in case a live conductor comes in contact with the line pole, cross-arm, clamp etc., sufficient amount of current must flow from the conductor to earth so that either the line fuse burns out or the circuit breaker trips out, thus causing the line dead. Hence, effective earthing ensures no risk of fire in case there is lightning stroke, short-circuit or any other fault in the line.

The ideal material for earth wire is the same metal of which the line conductor is made. But in practice almost everywhere galvanised iron wire is used as an earth wire. Continuous earth wire drawn along the line remains firmly fastened to every pole by bolts and nuts. If wooden pole is used in the line, the earth wire is looped at each bolt and dropped down for connection with the earth electrode below.

In case of a metal pole the part of the earth wire dropped down for connection to the earth electrode remains affixed to the pole just above the ground level by means of a bolt. This arrangement makes it convenient in subsequent line testing also.

The size of an earth wire depends on the metal of which the earth wire is made and also on how much current would flow through the earth wire in the event of a fault in the line. In low and medium voltage distribution lines normally galvanised iron wire of size 6 or 8 S.W.G. (4 mm dia.) is found to be used as earth wire.

If, however, the line current is very high, earth wire of size greater than this is also used. Wherever an overhead line is drawn across a roadway or a railway, the two poles on both sides of the roadway or railway must be connected to separate earth electrodes, although there may be a continuous earth wire drawn along the line. Besides, cradle guard, lighting arrestor etc. should also be separately earthed according to regulations.

Materials used as earth electrodes are galvanised iron pipe or rod, cast iron plate, galvanised iron strip and coil of galvanised iron wire. In distribution lines, however, use of first two materials is found to be much more than that of last two materials. Where moist soil is available after a little digging, galvanised iron pipe or rod or cast iron plate is used as an earth electrode there. But in a hilly region moist soil is not available even in a deep earth pit.

Hence, instead of making the pit deeper, glavanisech- iron strip may be used as an earth electrode there. It is always considerably less expensive to use a coil of galvanised iron wire. Normally, galvanised iron wire of size 8 S.W.G. is coiled into 115 turns, each of diameter 50.8 mm (2 inches), and is used as an earth electrode.

At first an earth pit of 2 metres deep is made ready at a distance of 90 cm (3 ft.) from the foot of a pole. The diameter of the pit is about 60 cm (2 ft). At the centre of this pit a galvanised iron pipe of diameter 5 cm and length 1.83 metres (6 ft.) is placed as an earth electrode.

The top of the electrode pipe remains about 20 cm below the ground level. To this end a solid galvanised iron wire of size 8 S.W.G. is fastened. The other end of the wire is drawn through the earth and taken to a point somewhat higher than the ground level. Here it is tightly fastened to the foot of the pole with the help of a bolt and nut.

In case of wooden pole this end of the wire remains clamped with the continuous earth wire running at the top of the poles. At first some quantities of charcoal and salt are dropped into the pit and then the rest of the pit is filled up with soil.

If cast iron plate is used as an earth electrode, its size should be 60 cm x 60 cm x 9 mm. The plate is vertically placed at the centre of the pit and galvanised iron earth wire is tightly-fastened to it at the central hole with the help of a bolt and nut.

Service Connection:

The separate branch line drawn from a distribution line for giving supply of electricity to a house or a factory is known as service connection. According to demand of electrical power service connection is given to a consumer either at low pressure or at medium pressure. Low voltage connection is given at 225-volt, two-wire (outer and neutral) d.c. or at 230-volt, two-wire (phase and neutral) a.c.

Usually medium pressure supply is given if the load demand exceeds 4 KW. In case of d.c. supply service connection is provided with three wires (positive, negative and neutral) and the supply pressure is 450 volts, while for a.c. supply service connection is provided with four wires (three phase wires and one neutral) and the supply pressure is 400 volts. Of course, along with every service connection a single earth wire is also to be drawn from the distribution line up to consumer’s main switch.

Service connection may be given either by overhead line or by underground cable. Connection by underground cable is, of course, more expensive. Hence, for supply of electricity to houses at village area and to small workshops, overhead service connections are usually provided.

Indian Electricity Rule No. 89 states that no service line or tapping shall be taken of an overhead line except at a point of support. A connection from any other part of the line is not permissible. In order to prevent the pole from bending, a suitable stay-set has to be installed to keep it erect. The distance between the pole and the pipe fixed to the wall of the consumer’s house should not exceed 35 metres (115 ft.).

Service line may be drawn in two ways. After installing service pipe in consumer’s house, connection may be given by drawing bare conductors from the pole. On the other hand a catenary wire is at first drawn between the pole and the service pipe and connection may also be given by insulated cable (P.V.C. wire or weather proof wire) which remains fixed with the catenary wire.

To provide service connection by bare conductors, three wires namely phase, neutral and earth are to be drawn from the pole up to consumer’s service pipe or bracket. All aluminium conductors are drawn for phase and neutral, while No. 8 S.W.G. galvanised iron wire is used as earth wire.

The configuration of a service line depends upon that of the distribution line. Fig. 291 shows how a service line is drawn from a distribution line in which line conductors are drawn in vertical configuration. To the shackle insulators of the phase wire and the neutral wire of the line from which service connection is to be taken, two pairs of shackle straps are to be fastened and to the free ends two shackle insulators of comparatively smaller size are fixed.

The wires of service connection remain tied to the grooved necks of these smaller insulators. The ends of the service wires, after coming round and being tied to the necks of the shackle insulators, are bent in the form of boxes. Main lines have connections with these boxes. This is not only convenient for connection but also ensures safety of the service lines.

On account of a fault when a wire snaps due to a flash, only box is affected but not the service line. One end of a small piece of wire connecting the main line with the service line is fastened to the main line by a Parallel Groove Clamp (in brief P.G. Clamp). If the main line is a live line, an aerial fuse is connected to the other end of the small connecting wire.

From this aerial fuse connection is taken and tied up to service line box by means of a suitable binding wire. In case of neutral line, however, no aerial fuse is used. Connection between a main line and service line by small piece of wire is known as Jumper Connection.

In consumer’s premises at the point where a service connection is to be taken, a galvanised iron pipe of diameter 4 cm (1 ½ inches) is fixed on the outer wall by means of clamps. Suitable stay-set is also used with the pipe to keep it erect. The top of the pipe is bent downwards. This prevents entry of rain water into the pipe.

Besides, a piece of wooden bush is fitted at the mouth of the pipe so that the insulation of the wires does not get damaged. The lower part of the pipe is taken inside up to consumer’s meter board through a hole made in the wall. For single-phase service connection a wooden meter board of size 35 cm x 20 cm x 4 cm (14″ x 8″ x 1 ½”) is fixed on the wall at a height of 1.5 metres (5 ft.) from the floor. House service meter is mounted on this board and the main switch of the house is placed above the meter.

The length of the service pipe attached to the wall of the house should be such that the clearance between lowest conductor and the ground must not be less than the minimum clearance as per I.E. Rules. Three galvanised iron clamps are fixed to the upper part of the pipe for drawing service lines. Earth wire is directly tied to the clamp. For the phase wire and neutral wire, shackle insulators of size 6.7 cm x 6.7 cm (2½” x 2½”) are fastened to the clamps with shackle straps. A line conductor is taken round the grooved neck of this insulator and its end is bent into the form of a box and well tied by binding wires.

Connections from service main up to meter board through G.I. pipe cannot be drawn by bare conductors. Insulated wires are to be used for this work. P.V.C. or V.I.R. cables may be used, but there is probability of early deterioration of insulation. On the other hand first cost of drawing weatherproof wire may be somewhat high, but the insulation of the wire lasts longer.

Insulation at the end of the cable is peeled off from certain length and the bare end is tied to the box end of the service line. P.G. clamp may be used for this job or it may be tied up by a binding wire. If the top most wire of the service line is earth wire, no lightning arrestor is required to be installed at the entry of the consumer’s premises. Horn-gap arrestor should, however, be used if phase or neutral wire is drawn at the top.

From the line an earth wire is drawn along the pipe line and it remains attached to the outer surface of the pipe line by means of earth clamps. End of the earth wire is connected to consumer’s main switch or earth terminal.

Before inserting the insulated service wires into the pipe leading to meter board of the house, these are turned into coils of a few turns. This creates a strong opposition to the high frequency surges in the event of a lightning stroke. As a result this surge cannot reach the consumer’s meter board.

Besides, at a distance of 75 cm (2 ½ ft) from the service pole and at the same distance from the service pipe of the house, two safety devices are fixed on the service line. When the live wire is accidentally snapped, this arrangement is necessary to make the wire dead before it reaches the ground.

To provide single-phase or three-phase service connection to a two-storeyed or three-storeyed building, often T-bracket is used. As this type of bracket is shaped like T’, it is called T-bracket. Generally this bracket is made of angle iron or G.I. pipe. At the wall of the house the base of this bracket is strongly built-in. Use of suitable stay-set would make it further strong and more durable. Sometimes swan-neck bracket is also used. Fig. 292 shows the use of T-bracket for service connection.

If the conductors of the distribution line are drawn in horizontal configuration, service line conductors should also be drawn in same configuration. Although in a few cases there may be exception to this rule. In horizontal configuration shackle straps of the service line cannot be directly fastened to the line, insulators.

In that case a separate bracket is affixed just under the cross-arm of the main line. Shackle straps are tied to this bracket and with these shackle insulators are fastened. There is also another bracket tied to the service pipe which is fixed with the wall of the house. Service line is drawn between these two brackets.

Jumper connections with the main lines on the side of the service pole and connections with the meter board on the side of the house are completed in the same manner as stated before. But the shape of the safety device is different in this case. Safety device in this line will be either ring-type or bracket type. Phase-type safety device will not do in this case.

Often cradle guard is arranged below a service line drawn in horizontal configuration. In that case earth wire is fastened to one side of the lowest cross-arm, and to the other side of the arm neutral wire is fastened. One end of a cross-lacing is directly tied to the earth wire, while its other end is tied to the neutral wire with the help of a reel insulator or egg-type insulator. Fig. 293 shows how a service line of this type is drawn.

The number of wires to be drawn in a 3-phase service line is five in all,—three phase wires, one neutral wire and one earth wire. If the line is in vertical configuration, the top most conductor should be the earth wire. In horizontal configuration also, it is possible to draw the earth wire at the highest level. A galvanised iron pipe of diameter 5.1 cm (2″) is attached to the wall of the house for service connection.

It is necessary that this pipe is sufficiently long so as to accommodate a number of wires, otherwise it will not be possible to maintain minimum required clearance between ground and lowermost conductor. Three-phase service connection in horizon­tal configuration is shown in fig. 294.

In these days, for the convenience of work as well as for the less cost, 230-volt service connection is provided with insulated wires instead of with bare conductors. These wires may be P.V.C. insulated or weather-proof. The cost of P.V.C. wire is less, but the insulation of weather-proof wire lasts much longer than that of P.V.C. wire. In the service line a single piece of bare G.I. wire remains fastened to the service bracket of the house at one side and to a distribution pole at the other side.

This wire is called catenary wire. It also serves the purpose of an earth wire of the service line. One end of the catenary wire is tied to the earth wire of the distribution line, while its other end is drawn along the service pipe up to consumer’s main switch and is connected to the outer cover of the main switch or to an earth terminal. From catenary wire since phase and neutral wires remain suspended by means of reel insulators, the service connection is known as catenary service connection.

It is required to observe a few points at the time of drawing catenary service connection:

(i) Any insulated wire drawn for a service line must be continuous and must not have any joint whatsoever;

(ii) The size of G.I. wire must not be less than 8 S.W.G. (4 mm diameter);

(iii) The phase and neutral wires of die service line must be drawn through reel insulators;

(iv) The distance between the distribution pole and the service pipe must not exceed 35 metres.

Now-a-days pole-fuse is used in place of aerial fuse while connecting the phase wire of the service line with that of distribution line. Pole-fuse is made of porcelain and it is that kind of fuse cut-out which is usually found as fuse kit-kat in the distribution fuse board or in the main switch of a house.

In case of an wooden pole the fuse base can be directly screwed to the pole. In case of steel or concrete pole a small wooden board is at first clamped to the pole and then pole-fuse is fixed on it. Bare conductor cannot be used where jumper connection is provided through a pole-fuse. Only insulated wire can be used in such case.

In place of reel insulators often phase and neutral wires of a catenary service connection can be drawn along a G.I. wire with the help of only link clips. This involves considerably less expenditure. However, in the interest of the quality of work, reel insulators should be used. P sketch of catenary service line is shown in fig. 295.